This invention relates to a process for continuously preparing dimethyl carbonate which is useful as a synthesis starting material for an aromatic polycarbonate and various chemical products. More particularly, it relates to a process for preparing dimethyl carbonate from carbon monoxide and methyl nitrite in a vapor phase in the presence of a platinum group metal solid catalyst which can be carried out industrially advantageously and separated continuously. Dimethyl carbonate is a compound extremely useful as a starting material for organic syntheses of medicines and agricultural chemicals, and for syntheses of aromatic polycarbonates and urethanes.
In the prior art, as a process for preparing dimethyl carbonate by contacting carbon monoxide and methyl nitrite to a platinum group metal solid catalyst in a vapor phase, the present inventors have proposed, for example, in Japanese Provisional Patent Publication No. 141243/1991 (which corresponds to U S. patent application Ser. No. 07/599, 134 now U.S. Pat. No. 5,162,563 or European Patent Application No. 90 311 469.2) or Japanese Provisional Patent Publication No. 139152/1992. The reaction itself disclosed therein is extremely excellent as a process for preparing dimethyl carbonate. However, in order to apply the reaction to the industries and to effect preparation of dimethyl carbonate to an industrial scale, it is required to develop a process which can conduct the reaction, separation and purification continuously and effectively.
As a process for preparing dimethyl carbonate from carbon monoxide and methyl nitrite, in addition to the process proposed by the present inventors, there has been proposed a process as disclosed in Japanese Provisional Patent Publication No. 181051/1985, but the disclosure is insufficient for using the process as an industrially continuous process.
As a conventional technique, in the process for separating a desired component from a mixed gas, there may be mentioned an absorption method using a solvent, a condensation method by cooling, a method of using an absorbant and a combination of these methods when the desired component has a high boiling point as in the present invention. In addition, there is a specific method such as a membrane separation, but in order to prepare dimethyl carbonate which should be prepared by a mass production with inexpensive using an industrial apparatus, it is required that the industrial apparatus should be simple, advantageous in energy efficiency and economically excellent.
A mixed gas obtained by the reaction disclosed in the aforesaid Japanese Provisional Patent Publication No. 141243/1991 mainly comprises a nitrogen gas, and in addition to dimethyl carbonate, contains carbon monoxide, methyl nitrite, nitrogen monoxide, methanol and a small amount of dimethyl oxalate.
In order to separate dimethyl carbonate and dimethyl oxalate from the mixed gas, the above mentioned methods are used, and of these methods, an absorption separation using methanol has most commonly used. In fact, a certain extent of separation efficiency can be obtained by the method, However, in order to heighten separation efficiency to a level sufficient for an industrial scale, there are inappropriate aspects that cooling is conducted at a significantly low temperature, a separation tower having high step number is required or a separation system should be made significantly high pressure.
Also, when separating dimethyl carbonate by distillation, it forms azeotropic composition with methanol and thus it should be finally separated from methanol. Considering this fact, it is clear that a methanol concentration in a separated material from a reaction gas mixture should preferably be low in view of energy burden. Thus, it cannot be considered that it is appropriate to absorb and separate dimethyl carbonate in the mixed gas by using methanol.